Planar waveguide devices are important components of optical fiber communication systems. Such systems typically comprise long lengths of fiber for transmission and often use planar waveguide devices to perform a variety of processes such as filtering, multiplexing signal channels, demultiplexing, compensating chromatic dispersion and compensating polarization dispersion.
An optical fiber is typically in the form of a thin strand of glass having a central core of circular cross section peripherally surrounded by concentric cladding glass. The core has a higher refractive index than the cladding so that the light is retained in the core by total internal reflection and propagates in a fiber mode. For long distance transmission the core dimensions are typically chosen so that the light propagates in a single circular mode.
A planar waveguide device, in contrast, is typically formed by thin layers of silica supported by a silicon substrate. The core is typically of rectangular cross section. The core region is formed, as by etching of a masked surface, into a patterned configuration that performs a desired function. In order to permit small radius curves, and thus compact functionality, the difference in refractive index of the planar waveguide core and the index of the cladding is typically substantially greater than the corresponding difference for optical fiber. The planar waveguide is said to be high delta where delta (Δ) is given by the core index less the cladding index, all divided by the core index.
Unfortunately there is a problem in coupling light from the transmission fiber into a planar waveguide. In addition to a mismatch in refractive indices, there is also a mismatch in core size. The core size for a typical optical fiber is significantly larger than the optimal core size for a single mode planar waveguide; therefore, their optical modes don't match because the field is more confined in the waveguide than in the fiber. As a consequence of these mismatches, direct coupling of a fiber to the planar waveguide would incur prohibitive insertion loss of the optical beam.
The conventional approach to this problem is to provide the planar waveguide with an enlarged end for receiving the fiber and to gradually (adiabatically) taper the waveguide core in the lateral direction to optimal size. The lateral taper is on the same plane as the waveguide optical circuit. This approach reduces insertion loss but unfortunately adiabatic lateral tapering is not efficient for high delta waveguides, and it requires substantial length.
A more process-demanding approach is to start with an expanded height waveguide at the fiber end, that is then vertically tapered down to the waveguide circuit level. This technique was proposed by Koch et al. (T. L. Koch, et. al., “Tapered Waveguide InGaAa/InGaAsP Multiple-Quantum-Well Lasers,” IEEE Photonics Letters Vol 2. No 2 February 1990; See also A. Mahapatra and J. M. Connors, “Thermal tapering of ion-exchanged channel guides in glass,” Opt. Letters, vol. 13, pp. 169-171, 1988, and Shani, et. al., “Efficient coupling of semiconductor laser to an optical fiber by means of a tapered waveguide on silicon” Applied physics Letters 55(23), December 1989). However, it requires a substantially large waveguide starting core height which takes a very long time to grow.
A 2-D tapered segmented waveguide was demonstrated by Z. Weissman and A. Hardy. This technique implements two-dimensional mode tapering by introducing gaps between the segments of the waveguides. (Weissman and A. Hardy, “Modes if Periodically Segmented Waveguide” IEEE Journal of Lightwave Technology 11: 1831-1838 (1993)). In effect, the total effective index of the guiding area is reduced. This approach is very effective in reducing the coupling loss. However, for high index waveguides, the core thickness is small compared to the fiber, therefore, mode matching via segmented tapes is hard to achieve, and a better matching with the fiber mode can still be achieved.
Accordingly there is a need for an improved arrangement to compactly and efficiently couple light propagating in an optical fiber into a planar waveguide.